The long-term goal of this project is to elucidate the detailed mechanism of action on a molecular level of the proteolytic enzymes involved in human blood coagulation and fibrinolysis by chemical means. Through the use of substrates, substrate analogs, and various types of reversible and irreversible inhibitors, we shall investigate the mechanism and stereochemistry of their enzymatic reactions. A related goal is to devise highly selective and efficient inhibitors of these enzymes, both for mechanistic purposes and for possible practical application in medicine. The enzymes presently under study are human thrombin, plasmin, and urokinase, as well as bovine trypsin and chymotrypsin, which we study as model enzymes. In order to provide materials for investigation, a further objective is to develop methods for the large-scale preparation of proteolytic enzymes involved in human blood coagulation and fibrinolysis, and to work out convenient methods for their assay. At present, our preparative research is devoted to the isolation of human factor X (Xa).